As soon as Novak Djokovic clinched his third U.S. Open title with an overhead winner, he fell flat on his back and smiled.

The on-court collapse is a common reaction to winning a Grand Slam, but in Djokovic’s case, his victory fall was made that much sweeter by the long, stunning fall that preceded it.

Djokovic, 31, beat Juan Martin del Potro 6-3, 7-6(4), 6-3 on Sunday to win the 2018 U.S. Open, his second major title of the season. Djokovic’s title, his 14th Grand Slam, caps his best two-month stretch—he won Wimbledon, Cincinnati and now the U.S. Open—since the first half of 2016. It was around that time two years ago, after winning Roland Garros, that Djokovic’s form began its mysterious decline.

Now, Djokovic is making a strong case for himself as the best player on the men’s tour at the moment. And here’s the scariest part: Djokovic, now tied with Pete Sampras for the third-most Grand Slam men’s singles titles ever, could very well continue to improve next season.

On Sunday, Djokovic—who will rise to No. 3 in the rankings, behind only Rafael Nadal and Roger Federer—frustrated del Potro with a vintage display of resilient defense, impeccable returning and a steady forehand, overcoming del Potro’s resilience and a partisan crowd to win his third singles title of the season.

After Djokovic won a tense first set, the two competitors battled for 95 minutes in a marathon second set, which ultimately proved decisive. After Djokovic and del Potro traded breaks, the Serb managed a crucial hold in the set’s eighth game, with del Potro leading 3-4. Djokovic saved three break points in a back-and-forth game that lasted 20 minutes.

In the ensuing tiebreaker, del Potro—playing in his first Grand Slam final since ’09, when he won the U.S. Open at age 20—initially grabbed the lead, but the Argentinian's forehand uncharacteristically let him down. With the score tied 4-4, del Potro missed three consecutive forehands, handing the tiebreaker and second set to Djokovic. Djokovic broke del Potro twice in the third set to seal his victory.

Del Potro’s roaring forehand is his biggest weapon, but it was no match for Djokovic’s impenetrable defense. Del Potro finished the match with 31 winners, including 16 from his forehand, but it would have been far more if not for Djokovic’s unassailable court coverage.

“I was playing almost at the limit all the time, looking for winners with my forehands, backhands,” del Potro said. “I couldn’t make it because Novak was there every time.”

It was a victory reminiscent of Djokovic’s peak form of 2015 and early 2016, when he held all four Grand Slams simultaneously and dominated the tour more than any player since perhaps Federer in ’06.

At that time, Djokovic seemed poised to challenge Federer’s record of 17 Grand Slam titles. But Djokovic’s stunning fall, which coincided with the dual revivals of Federer (now at 20 Grand Slams) and Nadal (now at 17), came fast and hard. The first sign of his decline was a shocking third-round defeat to Sam Querrey at Wimbledon 2016, when Djokovic still seemed invincible. He reached the U.S. Open final that September, losing to Stan Wawrinka, but in 2017 he failed to reach a Grand Slam final for the first time since ’09.

There were injuries—Djokovic shut down his ’17 season in July to rest his troublesome elbow—and there were “some other things that I was going through privately,” as he once put it, but the Serb’s sudden decline was inexplicable. Whatever physical and mental advantages the former No. 1 held over opponents, particularly in best-of-five Grand Slam matches, had seemingly evaporated overnight.

Djokovic’s initial return from his injury wasn’t pretty: In Australia, he unveiled a revamped service motion and an elbow injury that hadn’t fully healed. He exited in the fourth round to 21-year-old Hyeon Chung and underwent surgery in February. Then came a miserable Sunshine Swing—he lost in his first match at both Indian Wells and Miami—and suffered early defeats in Barcelona and Madrid. But steadily he started to improve, reaching the semifinals in Rome and the quarterfinals of the French Open, where he lost to heavy underdog Marco Cecchinato.

“You learn from adversity,” Djokovic said during Sunday’s trophy ceremony. “You learn when you’re down and when you have doubtful moments when things are not working out as you want them to.”

It takes just a few consecutive wins to change a narrative. And in July, Djokovic strung together seven straight victories to win Wimbledon, his first major title in more than two years. Since the start of July, he’s 22-1, his lone defeat to Stefanos Tsitsipas in Toronto. His 13 consecutive wins since that defeat feature an impressive series of conquests: Federer, Marin Cilic, Grigor Dimitrov and Milos Raonic in Cincinnati, followed by Richard Gasquet, Kei Nishikori and del Potro in Flushing Meadows. There's plenty of tennis to be played before the next Grand Slam event, but considering his current form and the state of the field, Djokovic is an early favorite to win the 2019 Australian Open.

We spend an untoward amount of time and energy these days debating whether post-prime athletes are back—think Tiger Woods after a couple standout rounds. So during these last two months, as Djokovic rounded into his best form in years, the question became inevitable: Is Novak Djokovic back?

“I don’t want to think about that level again because I feel like I’m on a whole new level,” Djokovic said of his former dominance. "I don't like to talk about, let's compare ourselves to this year or that year. I just like to be here now." And if this U.S. Open was any indication, that version of Novak Djokovic will suffice.